RK Racing Chain 4022-018ZG Silver Aluminum Rear Sprocket and GB520MXZ4 Chain Race Kit Fits: Yamaha YZ250F/YZ125 01-06/05-13 Race kit contains: 13 tooth chromoly front sprocket; 49 tooth silver 7075 T6 aluminum rear sprocket; and RK GB520MXZ4-114 link gold chain MXZ4 chains are made for the serious motocross racer looking for the lightest, strongest, non O-ring chain MXZ4 chains feature chromoly steel construction with seamless rollers and bushings, and oversized special alloy pins for added strength All components of the MXZ4 chain are heat treated using RK's exclusive HIT (Heat Induction Transfer) process and include a clip type master link